OT - Pulling Single Phase from 3 Phase Service

[Anonymous-0]

I have a remote irrigation system solely powered by a 3 phase service. The pump is setup with 60 amp slow blow fuses and draws 36 amps running a full load. I have a need to pull a 20A 220V circuit at that location. Can I use a 277V transformer to pull the 220 single phase from 2 of the 3 phase legs or will the unabalanced load cause problems? I have tapped into one of the legs previously to run a injection pump, but am concerned about potential voltage drop with the bigger circuit. Any advice would be greatly appreciated.

Thanks
Jim

 

[Randy S]

Yes you can use a transformer to step down the voltage to whatever you need. I do electrical maint for a Vvvvvery large transformer manufacter. All our over head cranes run on 460 and all have a step down transformer to reduce the control voltage to 120v. with no problems. just be sure to fuse the incoming voltage and the outgoing voltage for the safety of your equipment and self.

good luck

 

[John T]

Jim, Am I correct in assuming the 3 phase service you have is 480 Y 277 V 3 Phase 4 Wire Y grounded??? If thats so, its 277 volts from any leg to NEUTRAL and 480 VAC from line to line. That being said, if you need 240 VAC single phase, yes indeed you can theoreticaly (provided you can get a 277/240 transformer) use any of the three 277 legs (to NEUTRAL) and transform it down from 277 to 240 VAC single phase to run a 240 VAC single phase motor.

NOTE your question is NOT correct in claiming you use two legs of your service to get 277 volts, if the service is what I think it is, as noted above, its 277 volts from any leg to NEUTRAL and 480 line to line. The other alternative is to use a 480 to 240 volt single phase transformer and feed its input line to line (any 2 phases) for the 480 and use the 240 volt as the single phase output.

If your service is a 480 volt three phase three wire open floating Delta, THERE IS NO 277 VOLTS which is why when you mention 277 volts I took it to be a three phase four wire Y system which is 277 any phase to neutral but 480 line to line.

I prefer where possible to balance the phases as best possible but you can still get by using a single leg of 277 to Neutral (or 480 line to line) to transform down to 240.

You reallyyyyyyyyyy need to insure if the service is a 480 Y 277 volt three phase 4 wire or a 480 volt three phase 3 wire open floating delta (no 277 volts) ?????????????

John T long retired electrical engineer n a lil rusty SO NO WARRANTY LOL

 

[John T]

Jim, Am I correct in assuming the 3 phase service you have is 480 Y 277 V 3 Phase 4 Wire Y grounded??? If thats so, its 277 volts from any leg to NEUTRAL and 480 VAC from line to line. That being said, if you need 240 VAC single phase, yes indeed you can theoreticaly (provided you can get a 277/240 transformer) use any of the three 277 legs (to NEUTRAL) and transform it down from 277 to 240 VAC single phase to run a 240 VAC single phase motor.

NOTE your question is NOT correct in claiming you use two legs of your service to get 277 volts, if the service is what I think it is, as noted above, its 277 volts from any leg to NEUTRAL and 480 line to line. The other alternative is to use a 480 to 240 volt single phase transformer and feed its input line to line (any 2 phases) for the 480 and use the 240 volt as the single phase output.

If your service is a 480 volt three phase three wire open floating Delta, THERE IS NO 277 VOLTS which is why when you mention 277 volts I took it to be a three phase four wire Y system which is 277 any phase to neutral but 480 line to line.

I prefer where possible to balance the phases as best possible but you can still get by using a single leg of 277 to Neutral (or 480 line to line) to transform down to 240.

You reallyyyyyyyyyy need to insure if the service is a 480 Y 277 volt three phase 4 wire or a 480 volt three phase 3 wire open floating delta (no 277 volts) ?????????????

John T long retired electrical engineer n a lil rusty SO NO WARRANTY LOL

 

[Anonymous-0]

Adding load to one phase might cause a bit of voltage unbalance and a bit of voltage unbalance will cause the 3 phase motor to have 6 times that bit of current unbalance. But if the feed to the irigation is at higher voltage with the transformers local to the load, then that unbalanced load probably won't make too much unbalanced current.

Gerald J.

 

[Anonymous-0]

Thanks John T. and others.

John T. - yes the service is 480 Y 277 V 3 Phase 4 Wire Y grounded, as I have 277V from each leg to ground and 480V between legs. I'm thinking I want to go with the 2 wire 480V single phase to 240V transformer as theorically this should help balance the load. Would the 240V output on this type of transformer have a single 240V output or 2 - 120V? If a single 240V how would I wire to accomodate a 3 wire 120V,120V,nuetral type outlet? Thanks.

-Jim

 

[Anonymous-0]

Yes you can get 240 vac single phase from your 480y/277 service. You will need a control power transformer (CPT)480x240v primary, 240x120v secondary, a 600V on/off switch, and 2 fuses/fuse holders, and 12 Gauge wire and lugs, plus a nema 12 box if being used outside large enough to put the components inside. I recommend Square D parts, CPT - 5 kVA part #5S1F (20 amps output), Switch part #2510-KO2, 600V 20 amp fuses and fuse holders of your choice. Connect 1 wire to "A" phase and 1 wire to "C" phase and connect to the 480V side of your CPT. Install switch and fuses between CPT and your 480V source. A electrical supply house can supply you with these items.

 

[John T]

Jim, if you used a straight forward two in two out 480 (line to line) to 240 volt single phase two wire transformer NO IT WOULD NOT HAVE 120 AVAILABLE i.e. its NOT two legs of 120 like your typical home service.

In order to arrive at two legs of 120 i.e. a 120/240 volt single phase THREE wire service, the transformer you use would have to have TO BE CENTER TAPPED. If it were 480 volt single phase in and 120/240 volt single phase three wire out, you would take the center tap, connect it to a made or other grounding electrode (it becomes a "seperately derived service" like when you place a dry transformer in a building) and then feed a main breaker equipped service entrance type panelboard where the common Neutral and Ground Busses would be bonded together.

I guess all you need is to find a dry transformer of the appropriate KVA you require thats 480 VAC single phase in with a 120/240 VAC (center tapped) single phase three wire output,,,,,,,,,tie the center tap Neutral to a grounding electrode (i.e. ground it),,,,,,,,,,,feed a service entrance panelboard with those three wires (L1 L2 Neutral),,,,,,,Bond the Neutral and Ground buss bars inside the panel and bond the panels metallic frame to the ground buss.

John T Again a lil rusty n long retired EE

 

[Anonymous-0]

Typically such a control transformer comes with two 240 volt windings and two 120 volt windings so it can be wired for 240 or 480 on the high side and 120 or 240/120 on the low side. That makes it more versatile than having just one winding for each side though it makes it cost a few pennies more for the added lead wires and internal splices. It reduces the need for inventory by three part numbers. I'm sure a 480 to 240 without center tap exists too. So its important to be sure the low side can be run 120/240. Then its important to know that if you put all the 120 volt load on one side, it may fry the transformer unless you increase its capacity to handle that unbalanced load.

A line to line load will still unbalance the 480 currents, and so the motor terminal voltages and thus the motor currents which exagurate the unbalance and if too unbalanced, fry a winding of the motor.

Gerald J.